A method of filling a portion between a sample serving as an observation target and an objective lens of a microscope with a liquid to increase numerical aperture and obtain higher resolution has been employed in observation using the microscope. This method is called liquid immersion, and water, oil, glycerin, or the like is used as the liquid used for filling, that is, as a liquid immersion medium. In an ideal state, it is possible to obtain a higher resolution as a refractive index of the liquid immersion medium increases.
A technique that uses the liquid immersion method is illustrated in, for example, PTL 1 and PTL 2.